The dismantling of Boris Johnson’s imagined sylvan pathway across the Thames has been described as withering. It is. But it is something else as well. Margaret Hodge’s excavation of how the Garden Bridge project has got into its present, rickety state is starkly illuminating about Johnson’s time at City Hall, an eight year tenure characterised by arrogance, hypocrisy and a paucity of achievements for which he has yet to be held fully to account.

The former mayor himself refused to assist Hodge, a former chair of the Commons public accounts committee, informing her that he did not consider doing so a good use of his time. None of the other leading players in the shoddy saga Hodge describes emerge from her 40-page review unscathed, but their readiness to help her piece it together leaves them comfortably in credit by comparison with their former boss who, as has long been his habit, chose to dodge the risk of taking responsibility for his own weaknesses.

Johnson came to power in 2008 promising enhanced transparency at City Hall and helped by Evening Standard allegations that cronyism had resulted in public money being squandered by his main rival, the incumbent, Ken Livingstone. Now, Hodge concludes that the man who has become the UK’s foreign secretary presided over a shifty procurement process that has produced, in her words, “the perception that the whole project was owned and controlled by a small inner group” and seen £37.4m of public money already spent on a river crossing that might never be built. If it is cancelled, the bill could rise to £46.4. If it is to be completed, Hodge thinks the taxpayer could end up being hit for £200m.

Hodge’s Garden Bridge journey takes us away from the leafy visions of its showbiz champion Joanna Lumley and Johnson’s chortling anticipation of an ideal setting for a crafty smoke or snog and into an undergrowth that is as at best untidy and at worst reeks of rot: a business case for spending the money, whose rigour Hodge is notalone in doubting, was not put together until after key contracts to designer Thomas Heatherwick and engineer Arup had been awarded; Hodge believes aspects of the procedure were “in direct contravention” of those Transport for London (TfL), which was responsible for awarding the contracts, is meant to follow; “the evidence leads me to believe that the procurement options were intentionally developed to enable Heatherwick Studio to qualify,” Hodge writes.

It didn’t help that no one seemed entirely clear exactly what the bridge was for, other than to give Johnson a high-profile project he could boast about before leaving office. Part of the problem was that his use of what are called mayoral directions – a special powers that mean he can instruct TfL to take specific actions that lie outside his statutory mayoral transport strategy – brought huge pressures to bear on his own team and TfL officials to hurry things along.

Hodge writes, startlingly, that actions were taken “which have the appearance of manipulating the procurement to achieve a predetermined outcome”. She says that Johnson must ultimately be held responsible for this, but that those around him should have stood up to him. Here, I have some sympathy with those who worked with or for the former mayor. “Good old Boris” expects to get his own way, believes rules are for other people and is adroit at dodging blame when he’s found out for breaking them. The London mayors might lack muscle compared with counterparts elsewhere, but they have plenty of power to push people around without being restrained.

Where does Hodge’s review leave the Garden Bridge? She doesn’t directly say that Sadiq Khan, who commissioned her review, should scrap it. Rather, she says he should refrain from signing guarantees to safeguard its future until the Garden Bridge Trust has raised the money it needs to build it in the first place, and that she doubts its ability to do so. Her efforts will hardly make that fund-raising easier. The outlook for the Lumley-Heatherwick-Johnson wheeze does not look good.

That is a pity, in some ways. The concept is attractive and the intended location has had its sensible supporters. Had Johnson gone about realising the idea with proper rigour and transparency, which might have included partnering with private sponsors in a more productive way, it might still be possible to wish it well. Khan’s formal backing for the bridge might not have been unrelated to the Boris-loving Standard’s enthusiasm for it during last year’s election campaign. Since winning, there’s been a feeling he’s been stealthily arranging its failure. If that is so, the Hodge review may well enable him to complete that task.

On London is run by Dave Hill, formerly the Guardian's award-winning London commentator, and written by him and an array of fellow Londoncentrics. It aims to improve the quality of coverage of London politics, development and culture.